Frequency band filter



INPUT 5 I OUTPUT June 8, 1943. i H, SALINGER I 2,321,521

FREQUENCY BAND FILTER Ffiled Jan. 16, 1941 2 Sheets-Sheet 1 F|G.l v 2 INPUT T T OUTPUT C v 2 I07 l4. 5 I9 20 no INPUT S OUTPUT HS n k, as? -2A |s' FIG. 3

T21 22 25 2? INPUT v f i OUTPUT FI G.. 4

SALINGER f- .12 v BY INVENTOR series reactances.

Patented June 8, 1943 FREQUENCY BAND FILTER Hans Salinger, Fort Wayne, Ind., asslgnor to Farnsworth Television and Radio Corporation, a

corporation of Delaware Application January 10, 1941, Serial No.'3'l3,870

2 Claims.

This invention relates to four-terminal networks composed of transmission-line elements and particularly relates to such networks having filter or transformer characteristics.

Filtering and transforming networks of various types generally comprise shunt reactances and series reactances. For relatively low frequencies these reactances may be represented by lumped elements such as conventional inductance elements and condensers. At higher frequencies, in the order of 30 megacycles and above, however, such practice is no longer practical and it is preferable to employ transmission-line elements.

Consequently, it is necessary to provide transmission-line elements representing shunt and Various satisfactory methods of obtaining shunt reactances have been proposed but heretofore no satisfactory transmission-line element and incorporation thereof in a main transmission line to represent a series reactance has been provided.

The main disadvantage hitherto encountered in providin a series-reactance element in a main transmission line was that such an element could not be connected into the main transmission line without causing serious defects such as radiation losses, changes in transmission characteristics and the like.

The object of the present invention, there fore, is to provide a new and improved fourterminal network composed of transmission-line elements including a series-reactance element in which no radiation losses and no changes in transmission characteristics are incurred.

In accordance with the present invention there is provided a four-terminal transmission-line network comprising a coaxial main line having an inner conductor and ashield conductor joining the input and output ends of the network. There are also provided means connected to the main line and providing a surface constituting an extension of the surface of one of said mainline conductors whereby current waves travel a substantially longer distance along the surface of one of said conductors than along the other between the input and output ends of the network, thereby eiiectively providing a series reactance in the network,

For a better understanding of the invention,

together with other and further obicts thereof,, reference is had to the following descriptiontaken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings: Fig. 1 shows a conventional type of four-terminal network while Fig. 2 shows the same network composed of transmission-line elements in the conventional manner; Fig. 3 shows an embodiment of the invention while Fig. 4 shows its equivalent network; Fig. 5 shows preferred modifications of the embodiment of Fig. 3 while Fig. 6

shows a four-terminal network embodying the present invention and Fig. 7 shows its equivalent circuit.

, Referring now more particularly to Fig. 1 of the drawings, there is shown a single mesh of a conventional filter having input and output terminals as labelled, an input shunt capacitance element l, a 'series reactance 2 comprising inductance and capacitance elements as shown and an output shunt capacitance element 3.

Fig. 2 shows the equivalent four-terminal network composed of transmission-line elements constructed in the conventional manner in which reactance elements corresponding to the elements I, 2 and 3 of the circuit of Fig. 1 are indicated by the numerals IA, 2A and 3A respectively. The network of Fig. 2 comprises a main coaxialtransmission line including an outer shield conductor l0 and concentrically arranged therein an inner conductor II. The mainline is interrupted between points 5 and 6. Joined to the portion of the shield conductor l0 shown to the .left of point 5 there is provided a shield conductor l 2 and an inner conductor l3 which latter is connected to the inner. conductor ll of the main line at a point I4. The transmission line IA comprising the conductors l2 and I3 is designed to constitute a shunt capacitance element.

. Similarly, the portion of the main line to the right of point 6 is likewise provided with a transmission-line element having an outer conductor l5 and an inner conductor l6 constituting the transmission-line element 3A which, as said before, represents a shunt capacitanceelement.

For the purpose of providing a series-reactance element, it is conventional to provide a transmission-line element 2A comprising an outer shield conductor l1 and an inner conductor it 2A then through the inner surface of the shield conductor ll of line 2A to the junction point on the inner conductor ll of the main line, It may also be noted that at the point 20A at which a lead wire to the point 20 is connected, current also flows on the outer surface of the shield conductor IT. This current flows through distributed stray capacity to adjacent filter elements or to ground, hence constituting a radia tion loss and thereby also changing the trans mission characteristics of the four-terminal network which is highly undesirable.

Fig. 3 shows an embodiment of the present invention comprising a main transmission line.

having an inner conductor 2| and an outer shield conductor 22. For the purpose of providing a series-reactance element in the transmission line there is provided a hollow annular member 23, joined to the shield conductor 22 and constituting an. extension of the inner surface thereof. The member 23 shall, for sake of convenience, be referred to as a radial channel.

In operation, current flows along the outer surface of the inner conductor 2| and the inner surface of the shield conductor 22 from the input to the output end of the transmission line. For the purpose of determining the electrical effect of the member 23, voltage measurements may be made between points 25 and 26, and 21 and 28, respectively. It is then found that the voltage between points 21 and 28 is equal to the sum of the voltages between points 25 and 26 and the voltages between points 25 and 21. Due to the fact that the current traveling along the inner surface of the element 23 between points 25 and 2'! travels a substantially longer distance than the current along the inner conductor between points 26 and 28, the current through the outer conductor will be delayed in phase with respect to the current through the inner conductor and hence the voltage between points 21 and 28 will differ in phase from that between points 25 and 26 as if a lumped series-reactance element had been connected in one of the current paths.

Fig. 4 shows the equivalent circuit, like points being indicated by the same numerals as in Fig. 3 and Z representing the effective reactance of the member 23. The impedance of the structure shown in Fig. 3 can be calculated to be fi0hj n(P1) Y0(P2) n(m) 1(P| whereby h indicates the distance between the radial portions of the member 23, 1-1 the inner radius of the shield conductor 22, T2 the radius of the annular member 23,

w the frequency in radians per second, 0 the velocity of propagation, j the square root of minus 1, J0, J1, Y0, Y1 Bessel functions of order 0 and 1, first and second kind, respectively,

In practice, the radius T2 of the member 23 becomes rather large and therefore renders a rather cumbersome structure, particularly so if this element is to be incorporated in a network together with other transmission-line elements.

Therefore, the embodiment shown in Fig. 5 is preferred. This embodiment includes a main transmission line having a shield conductor 30 and an inner conductor 3!. The member gena pair of members 32 and 33, substantially concentric with the main line and extending throughout a substantial portion of the length thereof. The same fundamental considerations apply to thi embodiment as to that of Fig. 3 and similar voltage relations between points 25 and 26, and 21 and 28, respectively.

It may be seen from the drawings of Fig. 5 that the short radial channel comprising portions 32A and 33A of the members 32 and 33, respectively, is also a, part of the concentric transmission line formed by the members 32 and 33. The radial channel should be so designed that it has the same characteristic impedance Zo as the transmission line comprising the cylindricalportions of members 32 and 33. The radial channel also has an electrical length L which will add to that of the concentric portion of the transmission line. If n and n are the inner and outer radius of the short radial channel respectively, It

the distance between members 32A and 33A, then Z0 and L.may be found approximately from the following formulas:

An example of how this invention may be used in a four-terminal network is shown in Fig. 6. A

composite transmission line comprising inner of the inner surface of the shield conductor 46 erally indicated at 23A and corresponding to and an inner conductor A, the latter being an extension of the inner conductor 4|. The structure comprising transmission lines 44 and represents a shunt reactance across the input end of the main line comprising the inner conductor 4| and the shield conductor 42, since an input current flowing along the inner surface of the shield conductor 42 proceeds along the inner surface of the shield conductor 43A, the outer surface of shield conductor 46, the inner surface of shield conductor 46, finally to the inner conductor 4|A. Hence it may be seen that the shunt reactance across the input end of the main line comprises the series combination of transmission lines 44 and 45.

For the purpose of providing a series reactance, there is provided a concentric cylindrical structure 41 which is substantially identical with the structure 23A of Fig. 5 comprising members 32 and 33 with their respective radial portions 32A and 33A. For the purpose of providing an output shunt impedance, there is provided a transmission line element 48 in the conventional manner.

Fig. 7 shows the equivalent circuit of the transmission-line structure of Fig. 6, the equivalent lump impedancesbeing designated by the same reference numerals as the transmission line in Fig. 6. In the design of such a filter, it is necessary first to compute the impedance values which the transmission-line'elements 44, 45 and 48 respectively, represent.

If, for exwherein Z1 is the characteristic impedance of this line and L1 its length. Z1 is, as well known, given by the formula 21 =60 log,

wherein n is the radius of the inner surface of the shield conductor 43A while n is the outer radius of the shield conductor 46. The method of filter design will be evident to those skilled in the art.

Usually two characteristic image impedances. Za and Zb, are ascribed to a filter mesh as shown in Fig. 7. The main line of the wave filter should have at its output end, a characteristic impedance Zb and at its input end a characteristic impedance Z5. In order to minimize undesired reflections Z8 may be made equal to Zb in the desired transmission range of the frequency band. If this is not done, the filter will, at the same time, act as a transformer, stepping up or down the input voltage according to whether Zb is greater or smaller than Za respectively. While this effect may be undesirable in some cases, it can be made use of in the construction of ultra-high frequency transformers composed of transmission-line elements to which the filter action is only incidental.

If a single network mesh, as shown by Figs. 6

and '7. does not provide sufficient filtering or;

transforming action, further meshes may be added thereto. The characteristic input impedance ZnB of the second mesh should then be made equal to the characteristic output impedance Zbl of the first mesh connected thereto. The main transmission line may have any length between the filter sections, provided the whole structure is properly matched by the final load. However, the distance within a filter section, that is, the distance between points 50 and of Fig. 6, should be as small as possible as compared with the electrical wavelengths of the frequency band to ue transmitted.

While the transmission-line elements M, 5, 41 and 4B constituting shunt and series reactances have been shown as shortcircuited at their far ends for the purpose of preventing radiation losses, it may be understood, however, that these transmission-line ends may also be open-circuited or terminated by condensers or the like without departing from the scope of this invention. In some cases it is possible to leave these transmissiomline ends open-circuited without incurring losses as in the case of transmission-line elements IA and 3A of Fig. 2 where the center conductors l3 and H are not connected to their shields l2 and i5 respectively, and are much shorter than the latter, so that th shields can be mechanically closed without electrically short-circuiting the lines at their ends.

It may also be understood that the 'transmission-line elements representing shunt and series reactance may alsobe made tunable by providing movable plugs at the shortcircuited end thereof. In the case of the series reactance element, as shown in Fig. 5, it is also possible to make either or both of the radial .portions 32A or 33A respectively movable in axial direction so as to provide a tuning means. Other possibilities will beevident to those skilled in the art.

In the above there has been described a new and improved series-reactance' transmissionline element and its connection in a main transmission line without incurringradiation losses or changes. There has also been shown how this transmission-line element can be used in combination with others in a composite four-terminal transmission-line network comprising shunt and series-reactance elements to provide a network having filter or transformer characteristics, or both.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is,- therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A four-terminal transmission-line network comprising a coaxial main line having an inner conductor and a shield conductor connected 'to the input and output ends of the network, and means connected to said main line providing a surface constituting a physical extension of the surface of one of said main line conductors and having a configuration such that it presents a .pair of spaced cylindrical elements concentric with said main line conductors throughout a substantial portion of the length thereof, whereby current waves travel a substantially longer distance along the surface of said one of said conductors than along in the other between said input and output ends of said network effective ly to provide a series reactance in said network.

2. A four-terminal transmission-line network comprising a coaxial main line having an inner conductor and a shield conductor connected to the input and output ends of the network, and means connected to said main line providing a surface constituting a physical extension of the surface of one of said main line conductors and having a configuration such that it presents a pair of spaced cylindrical elements surrounding said main line conductorsthroughout a substantial portion of the length thereof, whereby current waves travel a substantially longer distance along the surface of said one of said conductors than along the other between said in put and output ends of said network effectively to provide a series reactance in said network.

HANS SALINGER.

GERTIFICATE OE CORRECTION. Patent No. 2,321,521. June 8., 19145.

HANS s LINGER.

It is hereby certified that error appears in the printed specification of the above' numbered patent requiring correction as follows: Page 5, sec- 0nd column, line h6,-claim' -1, after the word "along" strike out --in--;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this i2l+th day of August, A. D. 1915-.

Henry Van Arsc lale, (Seal) Acting Commissioner of Patents. 

